Ink jet head and method for the manufacture thereof

ABSTRACT

In order to provide a miniaturized ink jet head having a piezoelectric actuator  21  by which ink in a pressure chamber  3  is emitted and to improve its productivity and reliability, a vibration plate  22  is made up of two layers having different Young&#39;s moduli, i.e., a layer  27  having a smaller Young&#39;s modulus and a layer  28  having a greater Young&#39;s modulus. Further, the Young&#39;s modulus of each of the layers  27  and  28  is set at values ranging from 50 GPa to 350 GPa and the total thickness of the vibration plate  22  is set at values ranging from 1 μm to 7 μm.

TECHNICAL FIELD

The present invention relates to an ink jet head for use in ink jetprinters and to a method for the manufacture of such an ink jet head.The technical field of the present invention pertains particularly to anink jet head of the type of emitting ink by a piezoelectric actuatorhaving a structure-improved vibration plate.

BACKGROUND ART

In recent years, the ink jet printer has been used widely forbusiness/home use. In order to meet recent demands for noise reduction,printing quality improvement, et cetera, several methods have beenproposed for ink jet heads for use in ink jet printers. Generally, inkjet heads can be classified roughly into the following two types.

In the first type, a portion of a flowpath or a portion of an inkchamber is formed, as a pressure chamber, by a piezoelectric actuatorhaving a piezoelectric element. Then, a pulse-like voltage is applied tothe piezoelectric element, hereby causing the piezoelectric actuator toundergo deformation. As a result, the pressure chamber is so deformedthat its volume is reduced. This generates in the pressure chamber apressure pulse which forces droplets of ink to be emitted from a nozzlein communication with the pressure chamber.

In the second type, a heat generating resistor is disposed in aflowpath. A pulse-like voltage is applied to the heat generatingresistor. The heat generating resistor generates heat, thereby bringingthe ink in the flowpath to the boil to generate vapor bubbles. Dropletsof the ink are emitted from a nozzle by the pressure of the generatedvapor bubbles.

The present invention pertains to the first type. Therefore, the firsttype is further described in detail. Referring to FIGS. 9 and 10, thereis shown an ink jet head as an example of the first type. This ink jethead is provided with a head main body 101 in which a plurality ofrecessed portions 102 for pressure chambers are formed. Each recessedportion 102 has a supply opening 102 a for supplying ink and an emissionopening 102 b for emitting the ink. The recessed portions 102 of thehead main body 101 are arranged such that they are spaced at specifiedintervals in one direction.

The head main body 101 is made up of a pressure chamber component 105defining sidewalls of the recessed portion 102, an ink flowpathcomponent 106 defining a bottomwall of the recessed portion 102 andformed by lamination of a plurality of thin plates, and a nozzle plate113. Formed in the ink flowpath component 106 are an ink flowpath 107for supply which is connected to the supply opening 102 a of therecessed portion 102 and an ink flowpath 108 for emission which isconnected to the emission opening 102 b of the recessed portion 102.Each ink flowpath 107 is connected to an ink supply chamber 110extending in the direction in which the recessed portions 102 arearranged. The ink supply chamber 110 is connected to an ink supplyaperture 111 formed through the pressure chamber component 105 and theink flowpath component 106 and connected to an ink tank (not shown).Formed through the nozzle plate 113 is a nozzle aperture 114 connectedto the ink flowpath 108.

A piezoelectric actuator 121 is provided atop the pressure chambercomponent 105 of the head main body 101 in a corresponding fashion tothe recessed portion 102. Each piezoelectric actuator 121 has avibration plate 122 blocking up the recessed portion 102 of the headmain body 101 to form, together with the recessed portion 102, apressure chamber 103. This vibration plate 122 is common to all thepiezoelectric actuators 121, serving also as a lower electrode common toall piezoelectric elements 123 which will be described later. Eachpiezoelectric actuator 121 has a piezoelectric element 123 provided at aportion of the top surface of the vibration plate 122 corresponding tothe pressure chamber 103 and an upper electrode 124 provided atop thepiezoelectric element 123 for the application of voltage to thepiezoelectric element 123.

In the piezoelectric actuator 121, when a pulse-like voltage is applied,through the vibration plate 122 acting as a lower electrode and theupper electrode 124, to the piezoelectric element 123, the piezoelectricelement 123 shrinks in a direction perpendicular to its thicknessdirection, whereas neither the vibration plate 122 nor the upperelectrode 124 shrinks. As a result, a portion of the vibration plate 122corresponding to the piezoelectric element 123 is deflected and deformedby the so-called bimetal effect, being formed into a convex shape towardthe pressure chamber 103. This deflection/deformation generates apressure in the inside of the pressure chamber 103. By this pressure,the ink in the pressure chamber 103 is emitted outside from the nozzleaperture 114 by way of the emission opening 102 b and the ink flowpath108.

Recently, various attempts have been made for further improvements inorder to meet severe demands for size/weight reduction, drive voltagereduction, noise reduction, cost reduction, and improvement in inkemission controllability. With a view to achieving furtherminiaturization and high performance, there has been made the attemptthat the vibration plate and the piezoelectric element are formed ofthin films capable of easily being subjected to fine processing (capableof easily being down-sized and precisely processed).

However, if reduction in film thickness is tried by simply employingmaterials, shapes, and configurations of conventional piezoelectricactuators, this will produce problems such as the occurrence of crackingin the vibration plate, piezoelectric element, or upper electrode, filmdebonding, film expansion, at the time of manufacture, therefore leadingto the drop in ink jet head productivity.

Additionally, also at the time when the ink jet head is in use, suchsimple reduction in film thickness inevitably results in the drop inmechanical strength because the thickness of each portion is thin.Therefore, cracking is likely to occur in the vibration plate whichfrequently undergoes deformation, thereby reducing the life of the inkjet head. Therefore, there have been demands for the realization of anink jet head which is miniaturized and achieves high performance in inkemission amount controllability and, in addition, which provides longerlife because of excellent component strength and is easy to manufacture.

Bearing in mind the above points, the present invention was made.Accordingly, an object of the present invention is to provide an ink jethead of the type that ink in a pressure chamber is emitted by apiezoelectric actuator which is miniaturized and improved inproductivity and reliability as high as possible by providing a devisedstructure for a vibration plate of the piezoelectric actuator.

DISCLOSURE OF THE INVENTION

In order to achieve the above object, in the present invention thevibration plate is made up of at least two layers having differentYoung's moduli. Alternatively, the vibration plate is made up of atleast one compressive residual stress layer having a compressiveresidual stress and at least one tensile residual stress layer having atensile residual stress.

The present invention provides an ink jet head comprising:

a head main body with a recessed portion for a pressure chamber formedtherein, the recessed portion having a supply opening for supplying inkand an emission opening for emitting the ink; and

a piezoelectric actuator including a vibration plate blocking up therecessed portion of the head main body so as to form, together with therecessed portion, the pressure chamber, a piezoelectric element providedon a portion of a side of the vibration plate opposite the head mainbody and corresponding to the pressure chamber, and an electrode,provided at a side of the piezoelectric element opposite the vibrationplate, for the application of voltage to the piezoelectric element,wherein, when a voltage is applied, through the electrode, to thepiezoelectric element, the portion of the vibration plate correspondingto the pressure chamber undergoes deformation, thereby causing ink inthe pressure chamber to be emitted out of the emission opening;

wherein the vibration plate of the piezoelectric actuator is formed bylaminating together at least two layers having different Young's moduliin the thickness direction of the vibration plate.

As a result of such a structure, the vibration plate is composed of atleast two different materials. Therefore, when the layers of thevibration plate are formed, they produces different internal stresses(strains), and in the entire vibration plate the internal stresses(strains) are cancelled. As a result, excessive stress concentration tothe vibration plate, the piezoelectric electric element, et cetera canbe suppressed. Accordingly, even when the vibration plate and thepiezoelectric element are reduced in thickness, they are prevented fromcracking at the time of their film formation and when being used,therefore achieving improvement in productivity and reliability.

It is preferable that the Young's modulus of each of the layers of thevibration plate is set at values ranging from 50 GPa to 350 GPa. Thisnot only provides an amount of deflection sufficient enough to cause inkto be emitted but also makes it possible to provide a sufficientincrease in the generated pressure affecting the ink emission rate.Therefore, the ink jet head superior in ink emission performance will beobtained.

It is preferable that at least one of the layers of the vibration platenearmost the head main body is made of a material having ink corrosionresistance. As a result of such arrangement, even when the vibrationplate is constructed such that it is brought into direct contact withink, neither expansion/shrinkage nor deterioration by the ink occurs,and even when used for a long time, cracking or the like is unlikely tooccur.

It is preferable that the ink corrosion resistant material is made ofone of simple substances of copper, nickel, chromium, titanium,molybdenum, stainless steel, and tungsten, one of oxides, nitrides, andcarbides of the simple substances, or an alloy selected from a group ofalloys containing the simple substances, respectively. As a result ofsuch arrangement, the vibration plate which is thin but strong can beobtained easily and dissolution/corrosion caused by ink can be preventedwithout fail. Further, it is possible to sufficiently increase thepressure that is generated in the pressure chamber.

It is preferable that the total thickness of the vibration plate is setat values ranging from 1 μm to 7 μm. This is because if the totalthickness of the vibration plate is below 1 μm it becomes difficult tosecure the strength of the vibration plate and the pressure that isgenerated in the pressure chamber becomes insufficient, while on theother hand if the total thickness is above 7 μm there occurs filmdebonding or cracking at the film formation time and the amount ofdeflection necessary for the emission of ink cannot be obtainedsufficiently. Therefore, it is possible to improve the ink jet headproductivity and reliability as well as the ink emission performance toa further extent.

The present invention provides another ink jet head comprising:

a head main body with a recessed portion for a pressure chamber formedtherein, the recessed portion having a supply opening for supplying inkand an emission opening for emitting the ink; and

a piezoelectric actuator including a vibration plate blocking up therecessed portion of the head main body so as to form, together with therecessed portion, the pressure chamber, a piezoelectric element providedon a portion of a side of the vibration plate opposite the head mainbody and corresponding to the pressure chamber, and an electrode,provided at a side of the piezoelectric element opposite the vibrationplate, for the application of voltage to the piezoelectric element,wherein, when a voltage is applied, through the electrode, to thepiezoelectric element, the portion of the vibration plate correspondingto the pressure chamber undergoes deformation, thereby causing ink inthe pressure chamber to be emitted out of the emission opening;

wherein the vibration plate of the piezoelectric actuator is formed bylaminating together at least one compressive residual stress layerhaving a compressive residual stress and at least one tensile residualstress layer having a tensile residual stress in the thickness directionof the vibration plate.

As a result of such arrangement, in the case that the vibration plate isformed of the foregoing residual stress layers, the vibration plate willbe prevented from being formed by crystal growth in one direction,thereby relaxing strain generated from in-crystal defect and opening gapand suppressing the occurrence of film debonding. As a result, theacceptable good ratio at the ink jet head manufacture time will beimproved and, in addition, the ink jet head life will be increased.Accordingly, it is possible to achieve improvements in ink jet headproductivity and reliability.

It is preferable that the residual stress of the compressive residualstress layer of the vibration plate is set at 300 GPa or below, and thatthe residual stress of the tensile residual stress layer of thevibration plate is set at 200 GPa or below. The reason is that if theresidual stress of the compressive residual stress layer is greater than300 GPa, then the compressive stress is increased to an excessiveextent, resulting in the occurrence of cracking and debonding in thevibration plate. On the other hand, if the residual stress of thetensile residual stress layer is greater than 200 GPa, then the filmbecomes cloudy or is colored black, failing to become a normal mirrorfinished film and therefore being incapable of functioning as avibration plate. Accordingly, it is possible to maintain the performanceof an ink jet head at an excellent level while improving itsproductivity and reliability.

It is preferable that both of the residual stress layers of thevibration plate are made of the same material having ink corrosionresistance. As a result of such arrangement, even when the vibrationplate is constructed such that it is brought into direct contact withink, neither expansion/shrinkage nor deterioration by the ink occurs,and even when used for a long time, cracking or the like is unlikely tooccur. Moreover, the adhesion between the residual stress layers can beincreased to a maximum extent.

It is preferable that the ink corrosion resistant material is made ofone of simple substances of copper, nickel, chromium, titanium,molybdenum, stainless steel, and tungsten, one of oxides, nitrides, andcarbides of the simple substances, or an alloy selected from a group ofalloys containing the simple substances, respectively. As a result ofsuch arrangement, the vibration plate which is thin but strong can beobtained easily and dissolution/corrosion caused by ink can be preventedwithout fail. Further, it is possible to sufficiently increase thepressure that is generated in the pressure chamber.

It is preferable that the total thickness of the vibration plate is setat values ranging from 1 μm to 7 μm. As a result of such arrangement, itbecomes possible to secure the strength of the vibration plate as wellas to sufficiently increase the pressure that is generated in thepressure, and neither film debonding nor cracking occurs at the filmformation time. In addition, the amount of deflection necessary for theemission of ink can be obtained sufficiently. It is therefore possibleto further improve not only the ink jet head productivity/reliabilitybut also the ink emission performance.

The present invention provides a method for the manufacture of an inkjet head in which ink in a pressure chamber is emitted by causing avibration plate to undergo deformation by the piezoelectric effect of apiezoelectric element, the ink jet head manufacture method comprisingthe steps of:

forming on a substrate an electrode and the piezoelectric element in asuperposed manner with the electrode disposed nearer to the substrate;

forming on the piezoelectric element the vibration plate by laminatingtogether at least one compressive residual stress layer having acompressive residual stress and at least one tensile residual stresslayer having a tensile residual stress in the thickness direction of thevibration plate by a sputter technique;

adhering together the vibration plate and a pressure chamber componentdefining the pressure chamber; and

after the adhering step, removing the substrate.

Since the vibration plate is formed by sputtering such as high frequencysputtering, DC sputtering, et cetera, this makes it possible to performaccurate control of the film thickness of each layer by time management.In addition, it is possible to form the residual stress layers byperforming adequate control of the film stress by changing parameters,such as the substrate temperature, sputter gas pressure, sputter power,TS interval (the target/substrate distance), of various sputterconditions. At this time, none of film expansion, film debonding, andthe like will occur in components such as the vibration plate and thepiezoelectric element, as described above. Further, sputtering, beingsuitable for mass production, may be used to form not only the vibrationplate but also the electrode and piezoelectric element. Therefore, it ispossible to manufacture inexpensive ink jet heads at a greater yield inlarge quantities.

It is preferable that the residual stress of the compressive residualstress layer of the vibration plate is set at 300 GPa or below, and thatthe residual stress of the tensile residual stress layer of thevibration plate is set at 200 GPa or below. As a result of sucharrangement, it is possible to maintain the performance of an ink jethead at an excellent level while improving its productivity andreliability, as described above.

It is preferable that the compressive and tensile residual stress layersof the vibration plate are formed by control of the pressure of asputter gas. This makes it possible to perform control of the in-filmstress state in a much easier way, and the compressive and tensileresidual stress layers can be formed easily. Gas pressure control isdetermined by the amount of gas (for example, Ar gas) introduced and theamount of opening of an orifice of a vacuum pump. The operation isaccurately controllable and has repeatability, therefore improving theink jet head productivity to a further extent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an ink jet head according to a firstembodiment of the present invention when cut off in the crosswisedirection of a piezoelectric element (taken along line I—I of FIG. 3).

FIG. 2 is a cross-sectional view of the ink jet head of the firstembodiment when cut off in the lengthwise direction of the piezoelectricelement (taken along line II—II of FIG. 3).

FIG. 3 is a top plan view of the ink jet head of the first embodiment.

FIG. 4 graphically shows a relationship of the Young's modulus of avibration plate with respect to the maximum deflection amount and to thepressure generated in a pressure chamber.

FIG. 5A-5G is a schematic explanatory diagram of a method for themanufacture of the ink jet head of the first embodiment.

FIG. 6 is a partially enlarged top plan view of the ink jet head showingthe opening dimensions of a recessed-portion of a head main body.

FIG. 7 is a diagram corresponding to FIG. 6, showing an example in whichthe opening of the recessed portion of the head main body and apiezoelectric actuator are formed into an elongated circular shape.

FIG. 8 is a diagram corresponding to FIG. 1, showing an ink jet headaccording to a second embodiment of the present invention.

FIG. 9 is a cross-sectional view of a conventional ink jet head when cutoff in the lengthwise direction of a piezoelectric element (taken alongline IX—IX of FIG. 10).

FIG. 10 is a top plan view of the conventional ink jet head.

BEST MODE FOR CARRYING OUT THE INVENTION

EMBODIMENT 1

Referring to FIGS. 1-3, there is shown an ink jet head according to afirst embodiment of the present invention. The ink jet head of thepresent embodiment is provided with a head main body 1 with a pluralityof recessed portions 2 for pressure chambers formed therein, each of therecessed portions 2 having a supply opening 2 a for supplying ink and anemission opening 2 b for emitting the ink. The recessed portions 2 ofthe head main body 1 are each opened in one of outer surfaces (i.e., atop surface) of the head main body 1, being formed into a substantiallyrectangular shape, and are arranged at specified intervals in thecrosswise direction of the openings. Although in FIG. 3 only three ofthe recessed portions 2 (three of nozzle apertures 14, three ofpiezoelectric elements 23, and three of upper electrodes 24 which willbe described later) are shown for the sake of simplification, thesecomponents are actually provided in large quantities.

Sidewalls of the recessed portion 2 of the head main body 1 is formed bya pressure chamber component 5 of stainless steel or photosensitiveglass having a thickness of from 200 μm to 500 μm and a bottomwall ofthe recessed portion 2 is formed by an ink flowpath component 6 adheredto the pressure chamber component 5 and formed by lamination of aplurality of thin plates of stainless steel. Formed in the ink flowpathcomponent 6 are an ink flowpath 7 for supply connected to the supplyopening 2 a of the recessed portion 2 and an ink flowpath 8 for emissionconnected to the emission opening 2 b of the recessed portion 2. Eachink flowpath 7 for supply is linked to an ink supply chamber 10extending in a direction in which the recessed portions 2 are arranged.The ink supply chamber 10 is connected to an ink supply aperture 11formed through the pressure chamber component 5 and the ink flowpathcomponent 6 and connected to an ink tank (not shown). Provided on asurface (a bottom surface) of the ink flowpath component 6 opposite thepressure chamber component 5 is a nozzle plate 13 formed of anelectro-cast plate of stainless steel or Ni or of polymeric resin ofpolyimide, et cetera having a thickness of from 20 μm to 50 μm. Formedin the nozzle plate 13 is a nozzle aperture 14 connected to the inkflowpath 8. Each nozzle aperture 14 is disposed on a straight lineextending in the direction in which the recessed portions 2 arearranged.

Provided, in a corresponding manner to the recessed portion 2, on asurface (a top surface) of the pressure chamber component 5 of the headmain body 1 opposite the ink flowpath component 6 is a piezoelectricactuator 21. Each piezoelectric actuator 21 has a vibration plate 22which blocks up the recessed portion 2 of the head main body 1 so as toform, together with the recessed portion 2, the pressure chamber 3. Thevibration plate 22 is common to all the piezoelectric actuators 21 andserves also as a lower electrode common to all piezoelectric elements 23which will be described later. Each piezoelectric actuator 21 has apiezoelectric element 23 and a Pt upper electrode 24 having a thicknessof from 0.1 μm to 0.3 μm. The piezoelectric element 23 is provided, in acorresponding. manner to the pressure chamber 3, on a portion (a portionfacing the opening of the recessed portion 2) of a surface (a topsurface) of the vibration plate 22 opposite the head main body 1 and isformed of lead zirconium titinate (PZT). The Pt upper electrode 24 isprovided on a surface (a top surface) of the piezoelectric element 23opposite the vibration plate 22 for the application of voltage to thepiezoelectric element 23. The area of each of surfaces of the upperelectrode 24 in the thickness direction thereof is set slightly belowthat of the piezoelectric element 23 or may be made identical with thatof the piezoelectric element 23. Further, an insulator 25 formed ofphotoresist material or photosensitive polyimide is formed between theadjoining piezoelectric elements 23 and between the adjoining upperelectrodes 24.

The piezoelectric element 23 of the piezoelectric actuator 21 is applieda voltage through the vibration plate 22 as the lower electrode and theupper electrode 24. By the piezoelectric effect of the piezoelectricelement 23, a portion of the vibration plate 22 corresponding to thepressure chamber 3 deforms, thereby causing the ink in the pressurechamber 3 to be emitted out of the emission opening 2 b. In other words,when a pulse-like voltage is applied between the vibration plate 22 andthe upper electrode 24, the piezoelectric element 23 sandwichedtherebetween shrinks in the crosswise direction perpendicular to thethickness direction, whereas neither the vibration plate 22 nor theupper electrode 24 shrinks. Therefore, the portion of the vibrationplate 22 corresponding to the piezoelectric element 23 is deflected anddeformed by the so-called bimetal effect, being formed into a convexshape toward the pressure chamber 3. This deflection/deformationgenerates a pressure in the inside of the pressure chamber 3. By thispressure, a specified amount of the ink in the pressure chamber 3 isemitted, by way of the emission opening 2 b and the ink flowpath 8, tothe outside (onto a sheet of paper on which printing is performed) fromthe nozzle aperture 14. The ink thus emitted is deposited on the papersurface in the form of a dot.

Instead of emitting a single color of ink from the nozzle aperture 14,different kinds of ink colors such as black, cyan, magenta, and yellowmay be emitted from respective nozzle apertures 14 for achieving colorprinting.

The vibration plate 22 of the piezoelectric actuator 21 is formed bylamination of two layers having different Young's moduli, i.e., a layer27 having a smaller Young's modulus and a layer 28 having a greaterYoung's modulus, in the thickness direction of the vibration plate 22.In the first embodiment, the great Young's modulus layer 28 underliesthe small Young's modulus layer 27, being disposed nearer to the headmain body 1 than the small Young's modulus layer 28. Preferably, theYoung's modulus of each of the layers 27 and 28 is set at values rangingfrom 50 GPa to 350 GPa. The reason is as follows. If the Young's modulusof each of the layers 27 and 28 is set below 50 GPa, this results ininsufficient ink emission rates because the pressure generated in thepressure chamber 3 is low, although the amount of deflection necessaryfor achieving ink emission is sufficient, as shown in FIG. 4.Additionally, it is required to increase the total thickness of thevibration plate 22 above 7 μm, producing problems which will bedescribed later. On the other hand, if the Young's modulus is set above350 GPa, the vibration plate 22 comes to have difficulties in being bentalthough the pressure generated is increased to a sufficient extent, andsufficient deflection amounts cannot be obtained.

Moreover, it is preferable that the total thickness of the vibrationplate 22 be set at values ranging from 1 μm to 7 μm. The reason is asfollows. If the total thickness of the vibration plate 22 is set below 1μm, there are produced difficulties in securing the strength of thevibration plate 22 and the pressure generated in the pressure chamber 3becomes low. On the other hand, if the total thickness is set above 7μm, this may result in film debonding and cracking at the time of themanufacture of the ink jet head which will be described later and theamount of deflection for achieving ink emission is insufficient. In thecase that the total thickness of the vibration plate 22 is set at valuesranging from 1 μm and to 7 μm, the thickness of the piezoelectricelement 23 is also set preferably at values ranging from about 1 μm toabout 3 μm so that the piezoelectric element 23 is easily deflected.Desirably, the thickness of each of the small and great Young's moduluslayers 27 and 28 of the vibration plate 22 is set at values ranging fromabout 1 μm to about 3 μm.

Further, it is preferable that at least the great Young's modulus layer28 of the vibration plate 22 (i.e., the layer nearmost the head mainbody 1) is made of a material having ink corrosion resistance. The inkcorrosion resistant material is made of one of simple substances ofcopper, nickel, chromium, titanium, molybdenum, stainless steel, andtungsten, one of oxides, nitrides, and carbides of the simplesubstances, or an alloy selected from a group of alloys containing thesimple substances, respectively. Furthermore, it is preferable that thesmall Young's modulus layer 27 is also made of a material having inkcorrosion resistance different from the one forming the great Young'smodulus layer 28. Particularly, if the small Young s modulus layer 27 ismade of titanium (Young's modulus: 117 GPa) or copper (Young's modulus:124 GPa) and the great Young's modulus layer 28 is made of chromium(Young's modulus: 248 GPa), this provides the vibration plate 22superior in various aspects such as ink emission performance, strength,productivity, et cetera.

Next, a procedure of the manufacture of the above-described ink jet headwill roughly be described with reference to FIG. 5. In FIG. 5 thevertical positional relationship of the ink jet head is opposite toFIGS. 1 and 2. First, a Pt film 42 is formed all over a film formationsubstrate 41 of MgO by sputtering (see FIG. 5(a)). Following this, a PZTfilm 43 is formed all over the Pt film 42 by sputtering (see FIG. 5(b)).Then, the Pt film 42 and the PZT film 43 are patternized (i.e.,indivudualized) by RIE (Reactive Ion Etching) to form an upper electrode24 and a piezoelectric element 23, respectively (see FIG. 5(c)).Sputtering is the technique of forming a thin film by making utilizationof a phenomenon (called“sputter”) in which when a solid body (a targetbody) is radiated with high energy particles, target forming atoms areejected from the target surface. The sputter technique includes varioustypes such as high frequency sputtering, DC sputtering, et ceteradepending on the electrode structure and the way of generating particlesfor the sputtering. Any type of sputtering may be employed.

Thereafter, either photoresist material or photosensitive polyimideresin is filled between the adjoining upper electrodes 24 and betweenthe adjoining piezoelectric elements 23 by means of a spin coater toform the insulator 25 (see FIG. 5(d)). At this time, the top surface ofthe insulator 25 is made substantially coplanar with the top surface ofthe piezoelectric element 23 by a photolithography technique.

Next, the small Young's modulus layer 27 of the vibration plate 22 isformed, by sputtering, on the piezoelectric element 23 and on theinsulator 25. Following this, the great Young's modulus layer 28 isformed on the small Young's modulus layer 27 by sputtering thereby tocomplete the vibration plate 22 (see FIG. 5(e)).

Next, the great Young's modulus layer 28 of the vibration plate 22 andthe pressure chamber component 5 defining the pressure chamber 3 in thehead main body 1 are adhered together (wherein an aperture for thepressure chamber 3 is pre-opened) (see FIG. 5(f)). This is followed bymelting/removal of the film formation substrate 41 by thermophosphoricacid, KOH, or the like, and the ink flowpath component 6 and the nozzleplate 13 are sequentially adhered onto the pressure chamber component 5(see FIG. 5(g)). Prior to adhering together the great Young's moduluslayer 28 of the vibration plate 22 and the pressure chamber component 5,the ink flowpath component 6 and the nozzle plate 13 may be pre-adheredto the pressure chamber component 5.

Although not diagrammatically shown, the ink jet head is completed byproviding wiring to each upper electrode 24 and to the vibration plate22 and by performing other necessary processing.

When melting and removing the film formation substrate 41,thermophosphoric acid or KOH may reach and damage the piezoelectricelement 23 in the absence of the insulator 25. However, in the presentembodiment, by virtue of the insulator 25 and the upper electrode 24,the piezoelectric element 23 is prevented from being exposed tothermophosphoric acid or KOH.

Although the insulator 25 may be removed posterior to melting andremoving the film formation substrate 41, it is better to leave theinsulator 25 than removing it because of the following reasons (1) and(2). (1) Since the modulus of elasticity of photoresist orphotosensitive polyimide resin is not more than about 1/20 of that ofPZT (1/33 according to the measurement), the insulator 25 will notprevent the piezoelectric actuator 21 from operating even when theinsulator 25 is left intact.

(2) By virtue of the insulator 25, the piezoelectric actuator 21 can beprotected from mechanical external force resulting from some undesirablehappening or maloperation and, in addition, the transmission of stressbetween the vibration plate 22 whose modulus of elasticity is high andthe peripheral sidewall of the piezoelectric element 23 can be smoothed,thereby making it possible to improving the life of the piezoelectricelement 23.

In the first embodiment, the vibration plate 22 is made up of twolayers, i.e., the small Young's modulus layer 27 and the great Young'smodulus layer 28, having different Young's moduli (or made of differentmaterials) from each other. Therefore, when the layers 27 and 28 areformed, they exhibit different internal stresses (strains), and in theentire vibration plate 22 the internal stresses (strains) are cancelled.As a result, excessive stress concentration to the vibration plate 22,the piezoelectric element 23, et cetera can be suppressed.

For example, as shown in FIG. 6 (in which the vibration plates 22 areindividually provided for the respective piezoelectric actuators 21 andthe insulator 25 is not provided), in the case that the dimensions ofthe opening of each recessed portion 2 of the head main body 1 is 120μm×1500 μm, and that the vibration plate 22, formed to be slightlygreater than the recesses portion's 2 opening, is composed of onlychromium, the vibration plate 22 is distorted convexly to the sideopposite to the pressure chamber 3 (the upper side), and the maximumdistortion amount (the maximum warping amount) ranges from 0.5 μm to 1.5μm. On the other hand, if the vibration plate 22 is made up of twolayers, i.e., the small Young's modulus layer 27 of titanium and thegreat Young's modulus layer 28 of chromium, the maximum distortionamount ranges from 0.1 μm to 0.5 μm, thereby reducing the distortionamount of the entire vibration plate 22.

Further, as shown in FIG. 7, in the case that the opening of therecessed portion 2 of the head main body 1 is formed into an elongatedcircular shape (an elliptical shape) of about 250 μm (minor axis)×about500 μm (major axis), and that the vibration plate 22, the piezoelectricelement 23, and the upper electrode 24 are each formed into an elongatedcircular shape corresponding to the recessed portion 2, if the vibrationplate 22 is made of only chromium, the maximum distortion amount of thevibration plate 22 toward the side opposite to the pressure chamber 3becomes considerable great, i.e., from 5 μm to 15 μm, and on the otherhand, if the vibration plate 22 is made up of two layers, i.e., thesmall Young's modulus layer 27 of titanium and the great Young's moduluslayer 28 of the chromium, the maximum distortion amount becomesconsiderably small, i.e., from 0.5μm to 4 μm.

Accordingly, when manufacturing the ink jet head, none of cracking, filmdebonding, and film expansion will occur in the vibration plate 22 andthe piezoelectric element 23, thereby improving the productivity.Additionally, even when the ink jet head is used for a long time, thevibration plate 22, the piezoelectric element 23, et cetera are unlikelyto undergo cracking, thereby increasing the life of the ink jet head.These effects will be demonstrated more effectively when, as describedabove, the opening of the recessed portion 2 of the head main body 1 andthe piezoelectric actuator 21 are formed into an elongated circularshape.

In the first embodiment, the vibration plate 22 is made up of two layershaving different Young's moduli from each i.e., the small Young'smodulus layer 27 and the Young's modulus 28. However, the vibrationplate 22 may be made of three or more layers having different Young'smoduli from one another.

Further, in the first embodiment, the great Young's modulus layer 28 isdisposed nearer to the head main body 1 than the small Young's moduluslayer 27. On the other hand, the small Young's modulus layer 27 may bedisposed nearer to the head main body 1 than the great Young's moduluslayer 28.

EMBODIMENT 2

Referring to FIG. 8, there is shown a second embodiment of the presentinvention (in which the same components as shown in FIG. 1 have beenassigned the same reference numerals and therefore their descriptionwill be omitted), and in the second embodiment the structure of thevibration plate 22 of the piezoelectric actuator 21 differs from thefirst embodiment.

In the second embodiment, the vibration plate 22 is formed by laminatingtogether, in the thickness direction of the vibration plate 22, a singlecompressive residual stress layer 29 having a compressive residualstress and a single tensile residual stress layer 30 having a tensileresidual stress, and the tensile residual stress layer 30 is disposednearer to the head main body 1 than the compressive residual stresslayer 29. Preferably, the residual stress of the compressive residualstress layer 29 is set below 300 GPa (not less than − 300 GPa when thecompressive and tensile sides of the stress are represented by − and by+, respectively), while the residual stress of the tensile residualstress layer 30 is set below 200 GPa (not more than + 200 GPa when thecompressive and tensile sides of the stress are represented by − and by+, respectively). The reason is that if the residual stress of thecompressive residual stress layer 29 is greater than 300 GPa (i.e.,smaller than − 300 GPa), then the compressive stress is excessivelyincreased, resulting in breakage of the film formation substrate 41 andthe occurrence of cracking and film debonding in the vibration plate 22.On the other hand, if the residual stress of the tensile residual stresslayer 30 is greater than 200 GPa, then the film becomes cloudy or iscolored black, failing to become a normal mirror finished film andtherefore being incapable of functioning as a vibration plate.

It is preferable that both the compressive residual stress layer 29 andthe tensile residual stress layer 30 are made of the same materialhaving ink corrosion resistance (more specifically, the ink corrosionresistant material is composed of one of simple substances of copper,nickel, chromium, titanium, molybdenum, stainless steel, and tungsten,one of oxides, nitrides, and carbides of the simple substances, or analloy selected from a group of alloys containing the simple substances,respectively, as in the first embodiment), more preferably, chromium.Further, as in the first embodiment, preferably the total thickness ofthe vibration plate 22 is set at values ranging from 1 μm to 7 μm andthe thickness of the piezoelectric element 23 is set at values rangingfrom about 1 μm to about 3 μm.

A method for the manufacture of the above-described ink jet head will beexplained below. This manufacture method is the same as the firstembodiment except the formation step of the vibration plate 22.Therefore, only the formation step of the vibration plate 22 will beexplained omitting the overlapping description.

The insulator 25 is formed between the adjoining upper electrodes 24 andbetween the adjoining piezoelectric elements 23. Thereafter, thecompressive residual stress layer 29 of the vibration plate 22 is formedon the piezoelectric element 23 and on the insulator 25 by sputtering.Following this, the tensile residual stress layer 30 is formed atop thecompressive residual stress layer 29 by sputtering. When forming boththe residual stress layers 29 and 30 by sputtering, their film stresscan adequately be controlled by changing parameters, such as thetemperature of the film formation substrate 41, sputter gas pressure,sputter power, TS interval (the target/substrate distance), of varioussputter conditions. Particularly, if the sputter gas pressure iscontrolled, this makes it possible to achieve easy control of the filmstress.

More specifically, in the case that both the residual stress layers 29and 30 are made of chromium and a high frequency sputter device(frequency: 13.56) is employed, the compressive residual stress layer 29can be formed using the following conditions that the target diameter is8 inches, the sputter power is 500 W, the temperature of the filmformation substrate 41 is room temperature, and the sputter argon gaspressure is set at values ranging from 1 mTorr to 5 mTorr (from 0.13 Pato 0.67 Pa), and the tensile residual stress layer 30 can be formed whenthe sputter argon gas pressure is set at values ranging from 8 mTorr to12 mTorr (from 1.07 Pa to 1.60 Pa)

Further, in the case that both the residual stress layers 29 and 30 aremade of other than chromium, the value of the film stress with respectto the sputter gas pressure slightly differs from the above chromiumcase. However, basically, the relationship between the sputter gaspressure and the film stress is substantially the same as the abovechromium case, so that if the sputter gas pressure is controlled thismakes it possible to easily control the film stress of the residualstress layers 29 and 30.

The film stress values of the residual stress layers 29 and 30 can befound as follows. That is, a thin film is formed on a thin substrate (18mm×4 mm and 0.1 mm thick) whose Young's modulus and Poisson's ration areknown, the amount that the substrate warps is measured, and the filmstress of the thin film formed on the substrate is calculated from abending beam law relational expression to find values of the film stressof the residual stress layers 29 and 30. Whether the stress is acompressive or a tensile residual stress can be determined by whetherthe thin film formed on the substrate becomes concave or convex.

The optimum value of the thickness ratio of the compressive residualstress layer 29 and the tensile residual stress layer 30 correlates withthe opening shape (the length-width ratio) of the recessed portion 2 ofthe head main body 1, and it is sufficient that the film thickness ratioof the compressive residual stress layer 29 to the tensile residualstress layer 30 is so set as to range from ⅕ to ½ according to therecessed portion's 2 opening shape. If the film thickness of thecompressive residual stress layer 29 deviates from such a range andtherefore becomes excessively thick, components, such as the vibrationplate 22, the piezoelectric element 23, and the upper electrode 24,undergo cracking, film debonding, and film expansion when forming thevibration plate 22 and after removing the film formation substrate 41.This results not only in the drop in ink jet head productivity but alsoin the drop in ink jet head's mechanical strength when being used, whichmay lead to the drop in ink jet head's life.

In the second embodiment, the vibration plate 22 is made up of thecompressive residual stress layer 29 and the tensile residual stresslayer 30, because of which arrangement the vibration plate 22 will beprevented from being formed by crystal growth in one direction, therebyrelaxing strain generated from in-crystal defect and opening gap andsuppressing the occurrence of film debonding. As a result, theacceptable good ratio at the ink jet head manufacture will be improvedand, in addition, the ink jet head life will be increased. Therefore,the second embodiment provides the same operational effects as the firstembodiment. The formation of the residual stress layers 29 and 30 arecarried out by control of the sputter gas pressure in a sputtertechnique, thereby making it possible to easily and correctly controlthe in-film stress state of the residual stress layers 29 and 30, andthe vibration plate 22 can be formed easily at high yield.

In the second embodiment, the single compressive residual stress layers29 and the single tensile residual stress layer 30 are provided.However, any one of these layers 29 and 30 may be provided plurally orboth of them may be provided plurally. In this case, these pluralcompressive residual stress layers 29 or these tensile residual stresslayers 30 may differ in residual stress value from each other or may bethe same, and the order in which they are laminated is not limited to aparticular one. The residual stress layers 29 and 30 are not necessarilymade of the same material and may be made of different materials. Thecompressive residual stress layer 29 may be disposed nearer to the headmain body 1 than the tensile residual stress 30.

Further, in each of the first and second embodiments, the vibrationplate 22 is common to all the piezoelectric actuators 21. However, likethe piezoelectric element 23 and the upper electrode 24, the vibrationplate 22 may individually be provided for each piezoelectric actuator21.

Furthermore, in each of the first and second embodiments, the vibrationplate 22 serves also as a lower electrode. However, a separate lowerelectrode may be provided between the vibration plate 22 and thepiezoelectric element 23.

Additionally, in each of the first and second embodiments, the openingshape of the recessed portion 2 of the head main body 1 and thepiezoelectric element 23 of the piezoelectric actuator 21 are formedinto a rectangular shape. However, as described in the first embodiment,they may be formed into an elongated circular shape or an ellipticalshape or may be formed into other shapes.

Further, other various variations may be possible to make. For example,the piezoelectric element 23 of the piezoelectric actuator 21 and theupper electrode 24 may be different in material and thickness from thefirst and second embodiments and may be formed by other manufacturemethods. Further, the pressure chamber component 5 of the head main body1, the ink flowpath component 6, and the nozzle plate 13 may bedifferent in material and thickness from the first and secondembodiments.

INDUSTRIAL APPLICABILITY

The ink jet head and its manufacture method of the present invention areuseful when used in ink jet printers for computers, facsimile machines,photocopiers, et cetera. Particularly, the present invention is capableof miniaturizing ink jet heads and improving their productivity andreliability as high as possible and therefore its industrialapplicability is high.

What is claimed is:
 1. An ink jet head comprising: a head main body witha recessed portion for a pressure chamber formed therein, said recessedportion having a supply opening for supplying ink and an emissionopening for emitting said ink; and a piezoelectric actuator including avibration plate blocking up said recessed portion of said head main bodyso as to form, together with said recessed portion, said pressurechamber, a piezoelectric element provided on a portion of a side of saidvibration plate opposite said head main body and corresponding to saidpressure chamber, and an electrode, provided at a side of saidpiezoelectric element opposite said vibration plate, for the applicationof voltage to said piezoelectric element, wherein, when a voltage isapplied, through said electrode, to said piezoelectric element, saidportion of said vibration plate corresponding to said pressure chamberundergoes deformation, thereby causing ink in said pressure chamber tobe emitted out of said emission opening; wherein said vibration plate ofsaid piezoelectric actuator is formed by laminating together at leasttwo layers having different Young's moduli in the thickness direction ofsaid vibration plate.
 2. The ink jet head of claim 1, wherein theYoung's modulus of each of said layers of said vibration plate is set atvalues ranging from 50 GPa to 350 GPa.
 3. The ink jet head of claim 1,wherein at least one of said layers of said vibration plate nearmostsaid head main body is made of a material having ink corrosionresistance.
 4. The ink jet head of claim 3, wherein said ink corrosionresistant material is made of one of simple substances of copper,nickel, chromium, titanium, molybdenum, stainless steel, and tungsten,one of oxides, nitrides, and carbides of said simple substances, or analloy selected from a group of alloys containing said simple substances,respectively.
 5. The ink jet head of claim 1, wherein the totalthickness of said vibration plate is set at values ranging from 1 μm to7 μm.
 6. An ink jet head comprising: a head main body with a recessedportion for a pressure chamber formed therein, said recessed portionhaving a supply opening for supplying ink and an emission opening foremitting said ink; and a piezoelectric actuator including a vibrationplate blocking up said recessed portion of said head main body so as toform, together with said recessed portion, said pressure chamber, apiezoelectric element provided on a portion of a side of said vibrationplate opposite said head main body and corresponding to said pressurechamber, and an electrode, provided at a side of said piezoelectricelement opposite said vibration plate, for the application of voltage tosaid piezoelectric element, wherein, when a voltage is applied, throughsaid electrode, to said piezoelectric element, said portion of saidvibration plate corresponding to said pressure chamber undergoesdeformation, thereby causing ink in said pressure chamber to be emittedout of said emission opening; wherein said vibration plate of saidpiezoelectric actuator is formed by laminating together at least onecompressive residual stress layer having a compressive residual stressand at least one tensile residual stress layer having a tensile residualstress in the thickness direction of said vibration plate.
 7. The inkjet head of claim 6, wherein the residual stress of said compressiveresidual stress layer of said vibration plate is set at 300 GPa orbelow, and wherein the residual stress of said tensile residual stresslayer of said vibration plate is set at 200 GPa or below.
 8. The ink jethead of claim 6, wherein both of said residual stress layers of saidvibration plate are made of the same material having ink corrosionresistance.
 9. The ink jet head of claim 8, wherein said ink corrosionresistant material is made of one of simple substances of copper,nickel, chromium, titanium, molybdenum, stainless steel, and tungsten,one of oxides, nitrides, and carbides of said simple substances, or analloy selected from a group of alloys containing said simple substances,respectively.
 10. The ink jet head of claim 6, wherein the totalthickness of said vibration plate is set at values ranging from 1 μm to7 μm.